Cognitive disorders are observed in many neurological and psychiatric disorders, be they neurodegenerative (e.g. Parkinson's disease, Alzheimer's disease), neurodevelopmental (e.g. schizophrenia, autism spectrum disorders) or the consequence of other etiology.
Parkinson's disease is a progressive neurodegenerative disorder (synucleopathy) diagnosed on the basis of characteristic motor disturbances, asymmetry of symptoms onset and response to levodopa (Litvan et al., 2003). Lewy bodies, neurofibrillary tangles and plaques are observed in nigral, limbic and neocortical regions. These degenerations are supposed to affect catecholaminergic (dopamine and norepinephrine) and cholinergic neurotransmission. In particular, an important part of cognitive deficits (executive function and working memory) have been related to a decreased prefrontal dopaminergic signalling in non demented patients (Nandakumar et al., 2013).
Schizophrenia is the result of a complex series of neurodevelopmental or other changes that lead to impaired information processing in the brain (Marenco and Weinberger 2000). No single genetic change, aberrant protein function, or visible brain lesion has been demonstrated to lead to schizophrenia, and many different genetic and environmental changes are linked to increased disease risk (Fatemi and Folsom 2009). While many neurochemical signaling systems, such as the various monoamines, NMDA, and GABA, are likely to play a role in the etiology of schizophrenia (Pickard 2011), many pathways seem to converge on aberrant dopamine signaling as a final common pathway that leads to many of the observed symptoms (Howes and Kapur 2009).
With regard to the cognitive impairment, for which there is currently no treatment, patients with schizophrenia show significant deficits in specific cognitive domains, especially executive function, working memory, and episodic memory. Cognitive domains which are dysfunctioning in these two disorders are complex functions involving many neurotransmitters and brain regions; however, dopamine signaling in the dorsolateral prefrontal cortex (DLPFC) has been shown to play a critical role in these processes (Goldman-Rakic, Castner et al. 2004). One approach to rectifying cortical dopamine neurotransmission is to take advantage of the differential modes of clearance of dopamine from the different brain regions. In the midbrain, there is extensive expression of the dopamine transporter (DAT), which is thought to be primarily responsible for dopamine clearance from the synapse (Ciliax, Heilman et al. 1995). In contrast, cortical regions exhibit only low levels of DAT expression, and dopamine is cleared primarily by enzymatic catabolism of dopamine, with a contribution from the norepinephrine transporter (NET) (Yavich, Forsberg et al. 2007; Kaenmaki, Tammimaki et al. 2010). The primary enzymes responsible for dopamine catabolism in the prefrontal cortex (“PFC”) are monoamine oxidase (MAO) and catechol-O-methyltransferase (“COMT”).
Beyond Parkinson's and schizophrenia, inhibition of COMT may be useful in a number of neuro-psychiatric conditions, including ADHD, obsessive-compulsive disorder, alcoholism, depression, bipolar disorder (Lachman, Papolos et al. 1996), as well as age-associated cognitive symptoms, impulse control disorders, including compulsive gambling, sexual behavior, and other compulsive destructive behaviors. The role of COMT in dopamine metabolism in the brain make it an especially important target for improvement of cognition (Apud and Weinberger 2007).
Additionally, COMT inhibitors have shown utility in Parkinson's disease treatment, due to the role of COMT in metabolizing the dopamine precursor L-DOPA, which is given to Parkinson's disease patients to boost the levels of dopamine in the brain (Bonifacio, Palma et al. 2007). Since dopamine cannot cross the blood-brain barrier, L-DOPA is administered in its place and is transported into the brain and subsequently processed to dopamine. The percentage of exogenously administered L-DOPA that reaches the brain is ˜1%, and this low brain availability necessitates a high dose, which leads to peripheral side effects (Nutt and Fellman 1984). The primary enzymes responsible for dopamine metabolism are aromatic amino acid decarboxylase (AAAD) and COMT. Therefore, extensive efforts have been undertaken to develop potent and selective inhibitors of both enzymes. Carbidopa is an AAAD inhibitor now routinely given with L-DOPA, reducing the efficacious L-DOPA dose by 60-80% (Nutt, Woodward et al. 1985). Addition of a COMT inhibitor further decreases the variability of L-DOPA exposure, and a brain-penetrating COMT inhibitor could also increase brain dopamine levels.
Inhibitors of COMT have been developed for treatment of Parkinson's disease (Learmonth, Kiss et al. 2010). Notably, the nitrocatechol scaffold has been exploited to provide the clinically used drugs tolcapone and entacapone (Bonifacio, Palma et al. 2007). While they are effective in blocking peripheral COMT activity, entacapone has negligible brain penetration, and tolcapone has low but measurable levels in the brain (Russ, et al. 1999). Compounds with improved brain penetration should have greater efficacy for Parkinson's disease, as well as have utility for other psychiatric and neurological conditions such as cognitive impairment in schizophrenia. Despite the early clinical success achieved with tolcapone, the drug has been associated with serious liver injury, including three deaths, and requires strict liver function monitoring (Olanow and Watkins 2007). Thus, the risk-benefit profile for tolcapone prevents its widespread deployment, and new, inhibitors of COMT are needed, especially those that are active in the brain. Borchardt disclosed a series of non-nitrocatechol quinoline COMT inhibitors, but these compounds had weak potency (Borchardt, Thakker et al. 1976).
Accordingly, there remains a need for potent inhibitors of COMT and methods of using the same to treat central nervous system disorders.